1. Field of the Invention
The present invention relates generally to reflow systems for use in the manufacture of electronic circuit boards, and more particularly to a reflow system for heating a solder temporarily attaching electronic components to a circuit board, so as to melt down and then solidify the solder again to secure firm attachment between the electronic components and the circuit board.
2. Description of the Prior Art
Conventionally, a heating process is performed when electronic components, temporarily attached or tacked with solder to a circuit board, are to be firmly attached to the circuit board. In the conventional heating process, the circuit board is fed by a conveyor into a heating chamber of a reflow system so that the solder is melted with heat and then solidifies again, thereby firmly attaching the electronic components to the circuit board. In this instance, a gas in the heating chamber is heated by heaters at about 300.degree. C. so as to raise the temperature of the circuit board to a melting temperature (about 183.degree. C.) of the solder.
Since the gas in the heating chamber is heated at a very high temperature during the heating process, a foreign matter such as dust, adhered to the circuit board before the circuit board is loaded in the heating chamber, burns and gives out smoke. Smoke, however, exerts negative influence on the performance characteristics of the electronic components.
In order to remove the smoke, the conventional reflow system gradually discharges heated gas from the heating chamber together with the smoke. However, such a concurrent discharge of the heated gas and the smoke lowers the heating efficiency of the gas within the heating chamber and increases the running cost of the reflow system.
In addition, a flux is used to improve the wettability of the solder when the solder is melted down with heat. When the flux is subjected to a high temperature within the heating chamber in the reflow system, an organic solvent contained in the flux is vaporized and then adheres by condensation onto a surface of the circuit board. With this condensation of the organic solvent, the wettability of the solder is deteriorated considerably.
According to another known reflow system, a heating chamber is fitted with nitrogen gas so as to prevent the oxidation of metallic substances on the circuit board including solders, circuit patterns formed by a conductive metal, and electrodes or terminals of the electronic components. The nitrogen gas is heated at a high temperature and, in an atmosphere of heated nitrogen gas, a heating process is performed to firmly attach the temporarily soldered electronic components to the circuit board.
In the last-mentioned known reflow system, however, the outside air gradually flows into the heating chamber through an inlet and an outlet of the heating chamber with the result that the oxygen content within the heating chamber increases progressively. Under such condition, the metallic substances are susceptible to oxidation. In the case where nitrogen gas is discharged from the heating chamber to remove smoke and vaporized organic solvent generated during the high temperature heating process, the heating chamber must be replenished with nitrogen gas. The replenishment of nitrogen gas increases the running cost of the reflow system.